CriticalCVE-2023-24955

    Microsoft SharePoint Server Remote Code Execution Vulnerability (CVE-2023-24955)

    Published In CISA KEV since Sources: SingCERT, pwn.watch, CVE.org

    What is affected, what to do

    Microsoft SharePoint Server Remote Code Execution Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update Microsoft SharePoint Enterprise Server 2016 to version 16.0.5395.1000, Microsoft SharePoint Server 2019 to version 16.0.10398.20000, Microsoft SharePoint Server Subscription Edition to version 16.0.16130.20420. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft SharePoint Enterprise Server 2016 lt 16.0.0..16.0.5395.1000
    • Microsoft SharePoint Server 2019 lt 16.0.0..16.0.10398.20000
    • Microsoft SharePoint Server Subscription Edition lt 16.0.0..16.0.16130.20420
    Fixed in
    • Microsoft SharePoint Enterprise Server 2016 16.0.5395.1000
    • Microsoft SharePoint Server 2019 16.0.10398.20000
    • Microsoft SharePoint Server Subscription Edition 16.0.16130.20420

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    7.2

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    7.2

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: SingCERT, pwn.watch, CVE.org

    85.0 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.71

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    243.19 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: CSA/SingCERT How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    7.2 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    7.2
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    85.0 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.71
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · not automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 1 May 2024
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-94

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This CVE is on KEV with the ransomware-use flag and is actively chained with CVE-2023-29357, which lets an unauthenticated attacker reach Site Owner privileges and then invoke this bug to execute code on the farm server — network reach is enough, no user interaction required. Its exploitation likelihood ranks among the highest ever observed. Typical targets are internet-exposed on-prem SharePoint farms in public administration, universities and mid-market industrials. For NIS2-regulated organisations publishing SharePoint collaboration portals to the internet, treat this as P1; strictly internal, segmented farms with hardened Site Owner accounts can drop to P2 once the patch window and MFA coverage are confirmed.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy the Microsoft May 2023 security rollup: SharePoint Enterprise Server 2016 to 16.0.5395.1000, SharePoint Server 2019 to 16.0.10398.20000, SharePoint Server Subscription Edition to 16.0.16130.20420. Verify with Get-SPProduct and Central Admin → Patch Status.
      • Enumerate all Site Owner and Site Collection Administrator accounts and cut them to the strict minimum; disable service and legacy accounts holding this role until the farm is patched. The exploit chain with CVE-2023-29357 (spoofing to Site Owner) turns this into unauthenticated RCE, so trimming the role matters.
      • Audit internet reachability of SharePoint front-ends: until patched, allow TCP 80/443 only via an authenticating reverse proxy (Entra Application Proxy, WAF with pre-auth); remove direct internet publishing.
      • Preserve 90 days of IIS and ULS logs (%SystemDrive%\inetpub\logs\LogFiles, \15\LOGS\ or \16\LOGS\) off-host and triage for suspicious POSTs to /_api/web/, /_layouts/15/, /_vti_bin/client.svc before log rotation wipes evidence.
      • Rotate credentials for the SharePoint Farm Account, Application Pool identities and managed accounts after patching; if compromise is confirmed, also rotate farm passphrase and machine keys.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: place SharePoint front-ends in a dedicated VLAN with explicit egress; block outbound TCP 443 to non-business destinations at the perimeter to break second-stage downloads and C2.
      • WAF / reverse proxy: enable rules for anomalous payloads against _api/web/ and client.svc; block suspicious serialisation patterns (__type, System.Workflow, long Base64 blobs in POST bodies to SharePoint endpoints).
      • SharePoint least privilege: review permission inheritance across site collections, remove open „Everyone except external users" grants, restrict Site Owner to named admins, disable self-service site creation.
      • Endpoint hardening: ensure Microsoft Defender for Endpoint or equivalent EDR is active on every WFE and application server; enable ASR rules that block w3wp.exe from spawning cmd.exe, powershell.exe or script hosts even though SharePoint is a server workload — this is the single most effective webshell killer.
      • Configuration tightening: disable Developer Dashboard and custom script on site collections (Set-SPOSite -DenyAddAndCustomizePages), block SharePoint Designer organisation-wide, decommission SharePoint 2010/2013 workflow services if unused.
      • Backup and recovery: validate daily, offline or immutable backups of content databases and farm configuration — the KEV ransomware flag makes restorability a core control.

    3. Runbook step 3

      Detection rules

      • IIS access logs: POST requests to /_api/web/, /_layouts/15/toolpane.aspx, /_vti_bin/client.svc/ProcessQuery with unusually large cs-bytes, empty User-Agent, or scanner signatures. KQL shape: W3CIISLog | where csUriStem has_any ("/_api/web", "client.svc", "toolpane.aspx") and csMethod == "POST" and csBytes > 5000.
      • Sysmon EID 1 with ParentImage = w3wp.exe and Image in (cmd.exe, powershell.exe, pwsh.exe, cscript.exe, wscript.exe, rundll32.exe, mshta.exe) — on a healthy SharePoint farm this is practically zero-FP.
      • Sysmon EID 11 (FileCreate) under \TEMPLATE\LAYOUTS\, \inetpub\wwwroot\wss\VirtualDirectories\, \_catalogs\ for .aspx, .ashx, .asmx files not written by SharePoint setup or patch processes — likely webshell drop.
      • Windows Security EID 4688 / EDR process trees showing w3wp.exe writing to network shares or C:\Windows\Temp\, correlated with EID 4624 Logon Type 3 for the farm account.
      • ULS logs: repeated Runtime or General category entries with .NET deserialisation errors or Microsoft.SharePoint.WebPartPages stack traces. Sigma shape: logsource: product=sharepoint; detection: selection: Message|contains: ['BinaryFormatter','deserializ','InvalidCastException'] and Area: 'SharePoint Foundation'.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    Microsoft SharePoint Server Remote Code Execution Vulnerability

    Source: CSA/SingCERT, pwn.watch, NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    MicrosoftMicrosoft SharePoint Enterprise Server 2016
    • 16.0.0 – < 16.0.5395.1000
    • 16.0.5395.1000
    MicrosoftMicrosoft SharePoint Server 2019
    • 16.0.0 – < 16.0.10398.20000
    • 16.0.10398.20000
    MicrosoftMicrosoft SharePoint Server Subscription Edition
    • 16.0.0 – < 16.0.16130.20420
    • 16.0.16130.20420
    microsoftsharepoint_enterprise_server
    • 2016
    • —
    microsoftsharepoint_server
    • 2019
    • —
    • —
    • —

    Vendors and products

    Weakness classes (CWE)

    • CWE-94Improper Control of Generation of Code ('Code Injection')

      Base

      The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.

      Entry at MITRE CWE-94

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    CSA/SingCERTActive Exploitation of Critical Vulnerability in Microsoft SharePoint Server (opens in a new tab)CSA/SingCERTactively exploited

    References & sources

    • Microsoft Security Update GuideMicrosoft SharePoint Server Remote Code Execution VulnerabilityVendor advisory, Patch, Vendor advisory, pwn-watch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2023-24955government-resource, US government resource
    • pwn.watchpwn.watch: Microsoft SharePoint Server Code Injectionpwn-watch
    • NVD (NIST)NVD: CVE-2023-24955pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
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